A theoretical analysis of the vibrational modes of ammonium metavanadate

Vanadium(v) is an extremely rare and precious metal, mainly used in aerospace equipment and new energy construction. However, an efficient, simple, and environmentally friendly method for separating V from its compounds is still lacking. In this study, we used first-principles density functional theory to analyse the vibrational phonon density of states of ammonium metavanadate and simulated its infrared absorption and Raman scattering spectra. By analysing the normal modes, we found that the V-related vibration has a strong infrared absorption peak at 711 cm−1, while other significant peaks above 2800 cm−1 are from N–H stretching vibrations. Therefore, we propose that providing high-power terahertz laser radiation at 711 cm−1 may facilitate the separation of V from its compounds through phonon–photon resonance absorption. With the continuous progress of terahertz laser technology, this technique is expected to be developed in the future, and it may offer new technological possibilities.


Introduction
Vanadium(V) is an extremely rare metal with high melting and boiling points. It is resistant to both hydrochloric acid and sulfuric acid and outperforms most stainless steels in some ways. Because of its extensive utility, it is known as the metal 'vitamin'. V is mainly used in the production of high-strength low-alloy steels, specialised steels, and aerospace alloys, as well as in other applications. [1][2][3] In the eld of aerospace, the excellent improvement effect of V in titanium alloys has been discovered, making it useful in body structures and jet engines. [4][5][6] The global market for V redox ow batteries is also expected to grow at an annual rate of 59.7% between 2018 and 2025. 7 Considering the production and demand data of V for the energy technology industry specically, the demand for V is expected to increase by up to 73% by 2050. 8 This dramatic increase will be due to the growing demand for new materials, making mining and recycling of renewable resources strategically important for many countries.
Currently, there are various methods of extracting V directly from ores and secondary raw materials, including sodium roasting-water leaching V extraction 9-13 and calcication roasting-acid leaching V extraction. 14-18 However, these methods have disadvantages, such as causing pollution due to the use of various additives in the roasting and leaching processes and high costs. It is worth noting that China is one of the countries with the richest V mineral resources. 19 However, an efficient and environmentally friendly method of extracting V has not yet been developed, 20 and there is a lack of research on the recovery of secondary V resources, such as titanium-ferromagnetic slag.
Since the discovery of the crystal structure of ammonium metavanadate (NH 4 VO 3 ) in 1950, 21 many studies using infrared (IR) and Raman spectroscopy have focused on V in NH 4 VO 3 . [22][23][24][25][26][27][28][29][30] Among these studies, Waal and Twu et al. investigated the pyrolysis process of NH 4 VO 3 under the action of N 2 and NH 3 + H 2 O using Raman spectroscopy at the molecular level. 24,27 However, there has been a lack of theoretical study of the lattice dynamic processes based on vibrational spectroscopy. In this work, we simulated the vibrational spectrum of NH 4 VO 3 and analysed the normal modes. Through our analysis, we were able to assign V-related vibrational peaks and determine the IRactive modes of V in the NH 4 VO 3 spectrum. Based on these ndings, we propose a new method to assist V separation using photon-phonon resonance absorption (PPRA).

Simulation methods
We performed geometry optimisation and phonon calculations using the CASTEP code, 31 which implements the rst-principles density functional theory method. We adopted the generalised gradient approximation in the form of the revised Perdew-Burke-Ernzerhof (RPBE) exchange-correlation functional because the gradient of electron density in NH 4 VO 3 varies widely. 32 The convergence tolerance for energy and selfconsistent eld (SCF)was set to 1 × 10 −9 eV per atom to eliminate virtual frequencies. The energy cut-off was set to 750 eV and the K-point mesh to 3 × 1 × 2 to calculate phonons using a norm-conserving pseudopotential and the linear response method. The property of polarisability, IR and Raman spectra were calculated. The simulated spectra can be compared with experimental data and the vibrational normal modes from the phonon calculation can be used for assignments. NH 4 VO 3 has a pyroxene (Si 2 O 6 ) structure, and its primitive cell contains 36 atoms with space group Pbcm. 21  497 cm −1 to the symmetric vibration. 34,36 They tentatively assigned the peaks using group theory. Table 1 shows that the simulated normal mode at 489 cm −1 corresponds to a skeletal rotation, as shown in Fig. 3.
There are 10 normal modes ranging from 700 to 1000 cm −1 , which are all related to V-O stretching. Onodera and Park assigned three Raman peaks at 643, 895, and 925 cm −1 , and three at 646, 897, and 936 cm −1 , to asymmetric and symmetric V-O stretching vibrations, respectively. Onodera also observed IR peaks at 690, 850, and 935 cm −1 . Du assigned two IR peaks at 895 and 935 cm −1 to V-O stretching. Our simulations are in good agreement with the experiments. We found ve IR-active   bending vibration of the N-H bond. 30 Table 1 shows that the nine vibration modes from 1418 to 1673 cm −1 all represent NH 4 + bending.
In the higher-frequency range above 2800 cm −1 , the IR and Raman spectra exhibit several distinct characteristic peaks corresponding to N-H stretching vibrations. Waal et al. assigned vibrational peaks at 2839, 2926, and 3019 cm −1 to symmetric stretching, and peaks at 3122 and 3207 cm −1 to triply degenerate asymmetric stretching. 35

Conclusions
Based on density functional theory simulations of the VDOS of NH 4 VO 3 , we analysed the dynamic processes of the normal modes. The results show that the IR-active modes and Ramanactive modes are fully complementary. Each vibrational normal mode is either IR-active or Raman-active.
In particular, we conrmed that the normal modes from 711 to 994 cm −1 represent the V-O stretching vibrations. The highest-intensity peak in the IR spectrum is at 711 cm −1 , indicating that the PPRA effect of IR radiation at this frequency is very strong. Although there are still some high-intensity peaks in the region above 2800 cm −1 , they do not stimulate the PPRA effect of V-related vibrations.
V is typically obtained from V-bearing titanomagnetite and ilmenite ore 37 through metallurgical processing, where it is produced as a by-product. The V in the ore is usually in the form of powdered V 2 O 3 , which is then dissolved in water or an acidic or alkaline solution to form V-containing ion clusters. [38][39][40] The two main chemical methods currently used for the industrial extraction of V are the sodium roasting-water leaching process 9-13 and the calcium roasting-acid leaching process. 14-18 Based on our mode analysis, we propose the use of a high-power terahertz laser radiation at 711 cm −1 to assist in breaking the V-O bonds and separating V from NH 4 VO 3 . With the continuous progress of terahertz laser technology, this PPRA method could offer new application prospects. By utilising this PPRA physical method, it may be possible to achieve an environmentally friendly and efficient extraction of V from ores.

Conflicts of interest
There are no conicts to declare.